This invention relates to an audio amplifier and, more particularly, to an amplifier for amplifying an input signal to a predetermined value in order to sufficiently drive a load such as a loudspeaker.
It is a known technique to provide an audio amplifier having an output stage including two vacuum tubes connected in a push-pull configuration. In these known designs the vacuum tubes are usually in the form of triodes, diodes or pentodes, each of which is subject to one of the chief causes of tube failure, i.e. a control grid-to-cathode short largely caused by the fact that the control grid and the cathode of each tube are spaced relatively close together. Some prior designs, although recognizing the possibility of utilizing another grid as a control grid, suffer from other disadvantages. For example, in the latter designs, the output stage of the amplifier does not provide sufficient current to drive the latter grid in a manner to operate the tube in an efficient, linear manner without the need for feedback or other complicating techniques. Also, these latter designs are subject to reverse current effects caused by secondary electrons emitted by the plates being returned to the cathodes.
Other problems encountered in these types of prior art designs include the inability of the driver stage of the ampliier to supply a constant voltage between the grid functioning as the control grid and the cathode of the tube. Also, in many of these designs the electrode impedances are relatively high thus causing unwanted electron emissions and resultant thermal instabilities. Still further problems encountered with these types of amplifiers are the presence of relatively high idling currents and the requirement for relatively high bias voltges to turn off the tubes, especially when relatively high plate voltages are present.